I have an Iphone 3G with ios 4.2.1. I am using internet on my phone using Cellular data turned ON. It works fine for some time, but after then it disconnects and does not work untill i restart my phone. What is the issue.
Some phones do not re-login to the celluar network once disconnected. That's why a restart helps.
However, try "Reset Network Settings" on the iphone.